This invention relates to improvements in television receivers having two-way telephone communication capability.
Television receivers having two-way telephone communication capability are known in the art. U.S. Pat. No. 4,338,492 issued July 6, 1982 to Paul A. Snopko and assigned to Zenith Radio Corporation describes a commercially available system that enables a TV viewer to answer or initiate a telephone call through an operating television receiver. The receiver is equipped with a remotely operated switch enabling the receiver to be switched to a telephone mode. The telephone mode switching function is an added feature on a conventional hand-held wireless remote control unit for selecting and adjusting various television receiver functions.
A major limitation in such telephone arrangements results from the configuration of the telephone line, which is essentially two wires and an induction coil. Any system for supplying signals to the induction coil and for receiving signals from the induction coil will inherently experience a large amount of cross coupling. Prior art systems, including the Zenith system above, had to some degree succeeded in solving the problem of cross coupling, generally by including bridge circuit input/output arrangements and various controls for "tweaking" the telephone interface circuitry to particular line characteristics. Unfortunately, telephone lines differ in impedance and noise characteristics. While for any given set of line characteristics the interface circuit and amplifiers may be optimally adjusted to minimize cross talk, less than optimum performance will be obtained for different sets of line characteristics.
In a two-way telephone communication system through a television receiver the talk and listen channels are selectively operable, that is only one channel is operable at a time. When the talk channel is in use, the listen channel is disabled and vice versa. A channel switching arrangement controls channel switching with the strongest signal being the determinant. Difficulties with channel lock-out (a condition where the "non-operating channel" signal cannot overcome the "operating channel" signal) and too rapid switching of channels led to incorporation of compromise time constant networks to permit the non-operating channel to obtain control a predetermined time after the operating channel was no longer receiving a signal.
Another major problem is cross talk caused by a signal in one channel feeding into the other channel which also results in rapid switching and generally unstable conditions. To help counteract this problem a signal threshold is imposed on the disabled channel, requiring the signal in the non-operating channel to have a minimum amplitude to cause channel switching.
Difficulties are still encountered despite the use of circuits with time constants and thresholds. For example, large noises may cause channel switching and lock-out the other channel for a substantial time depending on the time constant of the circuit. In practice, a "listener" might cough loudly and switch channels, thus cutting off the "talker". The system would stay switched, keeping the talker cut off for a time period determined by the channel time constant and the signal level of the talker. Thus, conversation may be arbitrarily interrupted for a varying time period, resulting in not only unsatisfactory communication,, but also very unnatural communication.
Similar difficulties arise because of background noise, especially at the microphone end. Often the background noise level is sufficiently high to make it extremely difficult for the listening channel to become operative and also make it very annoying for the person at the other end of the telephone line to listen to the conversation. A mediocre or poor quality telephone connection can also to impose serious conversational difficulties, because of weak or noisy signals, in the listen channel. (To simplify terminology, the terms "listen channel" and "talk channel" are arbitrarily referenced to the television receiver location).
The listen channel is therefore preferably provided with greater sensitivity than the talk channel, the characteristics of which are much more controllable. However, since the signal level in the listen channel may not be representative of the signal level delivered from the speaker of the television receiver, optimally establishing the sensitivities of the talk and listen channels becomes extremely difficult. This, of course, is because the television speaker volume is primarily under the control of the viewer and only incidentally under the control of the other party on the telephone line.
Despite the various techniques mentioned above and the use of thresholds in an attempt to compensate for cross talk inherent in a telephone line, prior art systems often fall far short of desired performance levels.